The role of living roofs in holistic stormwater management systems - Living Roofs in Integrated Urban Water Systems - page 37

Each of the ield studies mentioned here also investigated suitability of
common ET models (equations) to predict ield measured living roof ET. ET is
rarely, if ever, directly measured in urban environments, rather it is calculated
from one or more well-known “potential” ET models developed from agricultural
sciences for crop production. Potential ET refers to the amount of ET that theo-
retically would occur if water were plentifully available, for example for irrigated
crops, whereas actual ET refers to the rate at which water is lost under existing
conditions. ET is a microscale process, thus the climate conditions in the immedi-
ate vicinity of the living roof are important, with emphasis on relative humidity,
solar radiation and wind speed. DiGiovanni et al. (2013) speciically identiied
that potential ET data from a regional weather station failed to capture actual
daily ET dynamics (possibly due to the difference in microclimate of the living roof
versus a weather station several kilometres away). Likewise, Voyde (2011) con-
cluded that none of 11 assessed models provided satisfactory prediction of actual
daily ET even using site-speciic climate measurements. In studies that identiied
periods of water limiting conditions, agricultural ET models overestimated actual
ET (Berretta et al. 2014; DiGiovanni et al. 2013; Voyde 2011). On the other hand,
these same studies, as well as Wadzuk et al. (2013), showed that variations of
the Penman-Monteith equation provide very good predictive capability for long-
term estimates when accounting for periods of water limiting conditions.
In agriculture, typical applications of ET models include crop coeficients to
adjust for species' speciic water use rates. Agricultural ET models have been
derived for and veriied on plants operating with C3 photosynthesis, thus it is
questionable whether they can relect the dynamics of living roof plant metabo-
lisms. Berretta et al. (2014) and Rezaei (2005) determined that with site-speciic
climate measurements, some models could be calibrated using crop coeficients
derived for some living roof assemblies, while Voyde (2011) did not. Wadzuk et
al. (2013) observed the deiciency of common ET models in replicating ield
measurements arose from the omission of accounting for growing media mois-
ture conditions. Rather than a crop coeficient, Stovin et al. (2013) determined
that a Soil Moisture Extraction Function (SMEF) proposed by Zhao et al. (2013)
could reasonably match living roof ET experimental data reported by Berghage et
al. (2007) and Voyde et al. (2010). Through the SMEF, the ratio of available
growing media moisture to the maximum water-holding capacity (ield capacity)
adjusts ET model estimates from common agricultural models in keeping with
observations that ET decreases as water becomes limiting. When applied in a
continuous simulation, the overall water balance model provided excellent results
to predict storm event and long-term stormwater retention.
Altogether, the question of appropriate quantiication of living roof ET is yet
to be resolved. Certainly variation amongst species and climate conditions are
factors to consider, but in an engineering context, it appears that the most inlu-
ential parameter is available growing media moisture at any given time, followed
by favorable climatic conditions (Schroll et al. 2011; Stovin et al. 2012; Voyde
2011; Wadzuk et al. 2013). Signiicant additional research is required in this area,